This invention relates generally to wind machines, and in particular to so-called horizontal-shaft wind machines.
A wind machine is a device for extracting energy from the wind. A typical horizontal axis wind machine consists of a pivoting platform mounted to the top of a tower. Attached to the pivoting platform is a rotor assembly joined to a power transmission system by horizontal shaft. The power transmission system is coupled to a work-performing device, for example an electrical generator or pushrod for pumping water. Wind machines can generally be divided into two categories: Lift-type and drag-type. Lift-type wind machines use slender airfoils similar to airplane propellers, which create lift as the wind passes over the airfoils to rotate the wind machine rotor assembly. As with aircraft propellers, the pitch of the lift-type airfoils must be matched to the windspeed for maximum efficiency and the pitch typically varies from the root of the airfoil to the tip in order to compensate for the different path velocity of the airfoil along the leading edge. Drag-type wind machines use relatively wide sails with large surface areas, which act to slow the wind striking the sails and convert a portion of the kinetic energy of the wind into rotary motion of the wind machine rotor. Drag-type wind machines typically include a rudder that extends from the rear of the wind machine head for aligning the wind machine so that the rotor always faces the wind.
Conventional wind machines, be they lift-type or drag-type typically comprise a single set of blades or a single rotor rotating about a horizontal shaft. Theoretically, the aerodynamic efficiency yielded by a single set of blades or by a single rotor cannot exceed 59.6% (See Betz, A. “Wind-Energie and Ihre Ausnutzun durch Windmuelen,” van den Hoeck & Ruprech, Goettingen, 1926). In practice, the output of typical single-bladed wind machines is substantially below 59.6%.
Multiple-rotor wind machines have been suggested as a solution to the limited aerodynamic efficiency of single bladed wind machines. U.S. Pat. No. 3,974,396 to Schonball discloses a lift-type windmill having two axially-displaced counter-rotating rotors in which one rotor drives the armature and the other rotor drives the stator of an electrical generator. This arrangement of counter-rotating blades enables the relative rotational speed between the armature and stator to be multiplied.
U.S. Pat. No. 7,384,239 to Wacinski discloses a lift-type windmill having two axially-displaced coaxial counter-rotating rotors each having two or more airfoils. The output of the counter-rotating rotors is combined through a planetary transmission to drive the single input shaft of an electrical generator.
U.S. Pat. No. 4,065,225 Allison discloses a multiple vane lift-type windmill having a plurality of axially-displaced rotors each having blades that are spring loaded to adjust the blade pitch as the rotor speed increases. The rotors of Allison are attached to a single shaft and do not counter-rotate. None of these patents suggest a drag-type wind machine in which the multiple rotors increase in diameter in a downwind direction, nor do they suggest a drag-type wind machine in which the rotors are angled rearward along a conical surface, nor a drag-type wind machine in which the sails have winglets formed on the leading and/or tip edges of the sails.